LOL... um. I think what you need to do is first find the difference (error) between your estimated weight and the actual weight.
So 60 - 68 = -8
Ignore the negative... so our error is 8.
Then to find the percentage error we do :
error/ actual value = 8 / 68 = 2/17
Then change it into a percentage by multiplying by 100.
2/17 * 100 = 11.76
Then round it to the nearest tenth ( 1 decimal point)
11.76 = 11. 8 (1dp)